Geochemical Journal, Vol. 44, pp. 461to 476, 2010**Present address: At - PDF document

Geochemical Journal, Vol. 44, pp. 461to 476, 2010**Present address: Atmosphere & Ocean Research Institute, The Uni-versity of Tokyo, 5-1-5, Kashiwanoha, Kashiwa, Chiba 277-8564,Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan.ronmental Earth Science, Hokkaido University, N10 W5, Sapporo 060-Copyright © 2010 by The Geochemical Society of Japan. the form of CH (Kvenvolden, 1988) and at least some in marine sediments seeps into the ocean and thusinto the atmosphere (Etiope and Milkov, 2004; Judd, 2004;., 2007). The mechanisms and fate of this seep-In particular, CH emitted from deep seafloor sources has budg-ets (Etiope and Milkov, 2004; Judd, 2004). Indeed, most would be oxidized within bottom water if verticaleddy diffusion in the water column is the only process upward (De Angelis Tsunogai ., 2000; Valentine ., 2001). The seafloordischarges of CH bubbles, however, can efficiently trans- from water depths greater than 2000 m to sur-huge discharges of CH due to massive decomposition ofOrigin and fate of deep-sea seeping methane bubbles at Kuroshima Knoll,Ryukyu forearc region, JapanU. T* A. KAKAYAMAOMATSUAMEYAMA F. NAKAGAWA and H. MACHIYAMAEarth & Planetary System Science, Faculty of Science, Hokkaido University, N10 W8, Sapporo 060-0810, JapanLaboratory for Earthquake Chemistry, Graduate School of Science, The University of Tokyo,7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, JapanJapan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, JapanReceived June 8, 2009; Accepted June 4, 2010640 m) using the gas-tight sampler WHATS attached to the . To evaluate the origin of the bubbles and verifywere analyzed. The major component of the gas bubbles was methane (C (67 16 ppmv) and helium(11 1 ppmv; was enriched relative to other hydrocarbons�) 3000). The (Ð40.1ä (Ð28.3ä (Ð28.0äof hydrocarbons produced by thermal decomposition of organic matter. The contribution of the mantle-derived (Ð19.1ä (Ð22.4ä (Ð19.9ästorage from Miocene in the gas reservoir. In addition, the anaerobic oxidation of CH within shallow sediments removedKnoll dissolved within 140 m of the seafloor. After the dissolution, the plume spreads horizontally along with the surfaceof equal density in the water column, while the concentrations decrease through dilution by eddy diffusion, rather than bying is estimated to be about 20% (IPCC, 2007). Enor- 462U. Tsunogai bubbles might also(Sills and Wheeler, 1992).In 1997, gas bubbling with large-scale chemosynthetic. 640 m), located in the forearc region of the Ryukyu (Machiyama O values of the carbonate crusts, Takeuchi developed under the area. The CHboth the bubbles and the carbonate crusts (Takeuchi ., 2007). They assumed that subtle changes in the ma- flux to the ocean andatmosphere. However, we had little evidence for the ex- discharge on the surrounding ma-In this study, we collected samples of both gas bub-Kuroshima Knoll (Fig. 2) using the WHATS gas-tightsampler (Water and Hydrothermal-fluid Atsuryoku TightFig. 1. Locations of Kuroshima Knoll (square), Taketomi spring (circle), and submarine Quaternary volcanic features (from Machiyama et al., 2001). The detailed bottom topography in the square (the area around Kuroshima Knoll) is presented in Deep-sea seeping methane bubbles in the Ryukyu forearc463Sampler) (Tsunogai to determine the ori- and discuss the possible sub-seafloor disso-ciation of methane hydrate using geochemical tracers. (i.e., biogenic from ac- reduction, thermogenic, inor-ble for their generation. Thus seepage gases could be usedet al., 2009), microbial alteration (Head et alBesides the gas bubbles and seeping fluid at theseafloor, the effluent plume in the water column was sam- bubbles in the ocean water col- oxidation on the fate of CH in the water column. in a CH ining the progress of microbial oxidation (Tsunogai preferentially, leaving the re- enriched in (Silverman and Oyama, does not vary through eddy diffu-effect (KIE) is around 1.005 at the bottom of the watercolumn (Tsunogai is defined as the ratio 1213 CH2OCO2HO CH2OCO2HO.Fig. 2. The bottom topography of Kuroshima Knoll and the locations of the gas seeping site (hydrocasts (, Stn. 01-2) (modified from Machiyama et al., 2001). 464U. Tsunogai in a CH decrease in the watercolumn was eddy diffusion (dilution) or oxidation.southern Ryukyu arc. The knoll has a flat table for its topat a depth of 1000 to 630 m, while the foot part reaches toa depth of 2400 m. On the basis of geological observa-covered from the knoll, Cenozoic sediments, such as the., 1998; Takeuchi The Cenozoic of the Ryukyu arc is divided into thelower Eocene Wano and Kayo Formations, upper EoceneMiyara Group, lower Miocene Yaeyama Group, SonaiTakarajima Group, Pliocene Shimajiri Group, middle toupper Pleistocene Ryukyu Group and Holocene coastaland terrestrial deposits (Nakagawa, 1983). The region ofthe southern Ryukyu arc, including the islands of Ishigakiactivity in the Miocene, such as the 21 Ma old graniticcanic breccia, tuff breccia, and lava flows along with smalldikes in Iriomote (Monden, 1968; Tiba and Saito, 1974).been found in the southern Ryukyu forearc area from theLarge-scale colonies of and spp. (both living and dead) (Okutani ., 2001). Active microbial oxidation of (both aerobic and anaerobic) has been found insediments just beneath the seafloor, especially in areasthat can be characterized by slow, diffusive seeping ofOn the basis of the tios (from Ð4.1 to Ð43.2ä(from +2.5 to +7.9ä) of the carbonates, Takeuchi have developed under Kuroshima Knoll. The CH Sample IDSampling locationDepthCH Lat. (N)Long. (E)m% VPDB ppmppm D1365 W12407.81212411.539635105 39.529428.31117.67111.2D1365 W22407.81212411.539635D1365 W32407.83312411.536637104 40.527928.33120.78213.1D1367 W22407.81212411.535635101 40.3317499.57Samples taken by M-type samplerD1357 M22407.80012411.53663819 41.8725D1364 M12407.82012411.57064217  Sample IDHeiso-C ppb/R Samples taken by D1365 W1210.4360.922.30.618.536628.322D1365 W2320.449D1365 W3360.4492.222.61.421.442428.819D1367 W26400.454Samples taken by M-type sampler Table 1. Chemical and isotopic compositions of the gas bubble samples, together with their sampling locations Deep-sea seeping methane bubbles in the Ryukyu forearc465O-enriched fluids likely derived from the partial disso-bles and carbonates on the seafloor (Takeuchi 70 m layer of the knoll, Takeuchi O values of the carbonates, however, there isGas bubbles and bottom fluid (seawater taken just of the JapanAgency for Marine-Earth Science and Technologyusing the WHATS (Tsunogai 2006). The sampler comprised four 150 cm stainless steelsample cylinders, eight ball valves, a motor-driven arm,a rail, a peristaltic pump, and a flexible Teflon tube con-nected to the inlet. The motor-driven arm on the rail isseawater that had filled the cylinder, we introduced sam-ple fluid/gas from the inlet into the cylinder. After com-the motor-driven arm again. At this stage, the next cylin-der is ready for sampling. The entire operation was con-In dives for sampling bottom fluid, a titanium inletseawater less than 30 cm above the seafloor, in areas mussels. A temperatureprobe was attached to the titanium inlet tube to measurethe temperature of the incoming bottom fluid. The bot-tom fluid samples, however, showed temperatures closecould not clarify the difference from the ambient seawater.In addition to samples taken by the WHATS, samples ofTable 2. Contents and stable carbon isotopic compositions of CH in the bottom fluid samples,together with their sampling locations, depths, and heights from the seafloor**Approximate sampling height from the seafloor. Sample IDLatitudeLongitudeDepth*Height**CH(N)(E)(m)(m)(mol/kg)(VPDBSamples taken by WHATSD1355W2246380.352D1355W4246410.319D1356W2246370.32.0 D1356W3246380.31.5 D1357W1246380.3662D1357W3246380.31.4 D1357W4246410.32.3 D1360W2246430.31.1D1363W2246360.3601D1363W4246350.355D1364W2246420.313D1364W4246420.31.5D1366W2246350.33.3 D1368W4246400.3165Samples taken by NiskinD1355N2246381.512D1356N1246381.51.3D1360N1246371.50.9D1360N2246361.50.9D1363N2246361.563 466U. Tsunogai bottom fluid were collected by Niskin samplers. The andoperated at the heights of 1.5 m from the seafloor. All thesamples of gas bubbles and bottom fluid are listed in Ta-bles 1 and 2, respectively., CO, and non-methane hydrocarbons), each, in the samples of bottom fluid, each fluid volume)pH of the fluid to less than 1). After waiting for gas ex-change equilibrium between the gas and the liquid phasebottom sampler was subsampled into a glass bottle (200displacement of pure water. Each sample of ambient bot-tom seawater collected by the Niskin sampler was slowly glass vials, for the determination (Tsunogai 2000). After approximately 3-fold volume overflow to of saturated HgClsolution (6 wt%) was slowly added to each vial as a pre-servative. The vial was then sealed with a gray butyl rub-Samples of effluent plume water supplied from theR/V Tansei-maru (JAMSTEC). While recording serialhydrographic data using a CTD system (Falmouth Scien-tific, Inc.), the samples of effluent plume were collectedbottles (General Oceanic Inc.). To analyze the concentra- glass vial. After approximately 3- solution (6 wt%) was slowly added asa preservative. To minimize air contamination with the solution, the solution was degassedimmediately prior to its addition. The vial was then sealeduntil analysis. The actual sampling depth of each Niskinods (Carpenter, 1965).NALYTICALcluding the gas phase samples extracted from the bottomfluid samples) were determined by continuous-flow iso-University (Tsunogai ., 2005). The concentrations ofFig. 3. The C) ratios of the Kuroshima bubble sam-microbial alteration during storage in gas reservoir estimated; see the main text for details). Approximatesource regions of thermogenic hydrocarbons and biogenic hy-drocarbons are also plotted, together with a line for hypotheti-cal mixing line between them with representative compositions(see the main text for details) (modified from Bernard et al., Deep-sea seeping methane bubbles in the Ryukyu forearc467., 1976). Thehelium and neon contents and He ratio in the gasIII) noble gas mass spectrometer in the University of To-cedure described by Aka the interlaboratory helium standard (HESJ, Matsuda ESULTSTable 1 and presented in Figs. 3, 4, and 5. The samples ofgas bubbles taken by the WHATS (D1365W1, D1365W2,W1365W3, and D1367W2 in Table 1) were composed (C while the concentrations of Nand CO were detected; less than 100 ppmv and less than3 ppmv respectively. Non-methane hydrocarbons were 20 ppmv for ethane (C 14 ppmv for pro- 0.9 ppmv for isobutane (iso-C 0.04 ppmv for 0.002 ppmv for). The C ratio is greater than 9. TheC values of the non-methane hydrocarbons were char- for C 2.2 for C for 2.0 for iso-C 0.4 for 1.5 for iso-Cby both helium enrichment (9.6 to 13.1 ppmv) and highNe ratios (from 15 to 545) in comparison with val-ues for air, indicating that no air contamination orfractionation affected bubble samples taken by theWHATS. Thus the concentration and isotopic composi-seafloor. On the other hand, the gas samples taken by theM-type sampler (D1357M2 and D1364M1 in Table 1)were rather different from those taken by the WHATS,Fig. 4. The helium isotopic compositions of the bubble sam-Mamyrin and Tolstikhin, 1984), pore gases in deep sedimentstaken around the Japan islands during the Deep Sea DrillingProject Leg 87 (Sano and Wakita, 1987), and seafloor seepinggases at the Taketomi spring (Oomori et al., 1993). Hypotheti-(mantle, crust, sediments, and Kuroshima bubbles) are alsoFig. 5. The carbon isotopic compositions of hydrocarbons inthe bubble samples taken at Kuroshima Knoll (: D1367W2) plotted as functions of the reciprocal of carbonnumber, together with compositions for the Sirius-1 ExmouthPlateau, offshore Australia, where anomalous carbon isotopiccompositions have been reported (James and Burns, 1984). 468U. Tsunogai vol% and 25 vol%, respectively. We concluded that airThus, we do not consider them in the discussions thatfollow. To determine the concentration and isotopic com-position of a gas sample taken from a deep seafloor, theAnalytical results for the bottom fluid samples, whichmight represent a mixture of bottom seawater and seep-ing material in the knoll, are presented in Table 2. Theyless than 0.3 m from the seafloor) by the WHATS. The almost corresponds to theC, respectively). This was, however, the major dissolved com-in comparison with seawater. If the site would be an ac-tive seeping site of some liquid material from deep sub-seafloor, it is difficult to assume such liquid having littleto seawater. The seeping material seems to be limited togas bubbles and liquid a few amount in the knoll. That is,simple upward migration of the gases from sub-seafloorgas reservoir and mixing into bottom seawater and/ornism to explain the CH-enrichment in the bottom fluidWe found clear enrichment of CH in the water col-umn at depths between 536 m and 644 m, the potential (Fig. 6). On the other hand, at depths less than500 m. The maximum enrichment was 82.0 nmol/kg at adepth of 615 m for station 01-0 (bottom depth of 640 m),6). In addition, we found smaller but clear anomalies atwhich were about 3 km horizontally from the gas seeping. 1050 and 870 m, respectively and thus it is un- (or CHfluid) emitted from adjacent seafloor was the cause of enrichment at stations 01-1 and 01-2. The lateral plume from station 01-0 (gas seep- in station 01-0 from theFig. 6. Depth profiles of (a) temperature, (b) potential density (the specific gravity anomaly), (c) methane concentration, an) in the water columns around the Kuroshima Knoll. The symbols in (c) and (d)are the same as those in Fig. 2. The shaded zone corresponds to the depths where the effluent plume had been detected. The dottlines in (c) and (d) denote the background profiles. Deep-sea seeping methane bubbles in the Ryukyu forearc469than 140 m from the seafloor, probably owing to com-plete dissolution of the gas bubbles to that height. AfterInitial generation of hydrocarbons in the bubblesdrocarbons. Specifically, we can differentiate thermogenichydrocarbons (generated by thermal degradation of or-ganic matter) from biogenic hydrocarbons (generated bymicrobial degradation of organic matter) by plotting theconventional diagram (Bernard ., 1977). The appli-cation of this diagram in the identification of surface gasseepage, however, provides misleading information for(Tsunogai ). Approximate source regions of both thermogenica hypothetical mixing line between them having respec- = and., 1977). While 0.5suggest thermogenesis. It is difficult to explain this con-tradiction through mixing between microbial and ther-theoretical mixing line. Rather, there was significant sec- under anaerobic/anaerobic condition. Tsunogai ported up to 50C-enrichment in residual CH throughsediments. The carbonate crusts including chimney-like and subsequent carbonate precipitation CHSOHCOHS+HO 2HCOCOCO+HO3232 CaCOCaCO., however, results in the., 1986), and thus it is difficult to explain the ob- in requires more than 90% oxidation of CH through the re- relative to CH (Table 1), suchenormous oxidation in the bubbles is unlikely.through the thermogenic process. That is, the removal of enrichment through molecular fractionation is ubiq-uitous for natural hydrocarbons (Nagy, 1960; Price andSchoell, 1995; Mango, 2001; Snowdon, 2001). There is than C (C + iso-Condary alteration for the hydrocarbons. Regarding ther-carbon reflects the integrated kinetic isotope effect dur-ing the cleavage of chemical bonds from parental mate-rial. This begins with long-chain organic matter and thusarrow in Fig. 5. The and in thebubbles are larger than the values expected for C and produced from sedimentary organic matter (Taylor process is fractional crystallization during hydrate for-molecules. While the structure I is usually pure CH 470U. Tsunogai hydrocarbons as significant components (Sloan,1998). The molecular distribution of the bubbles couldforming the structure II/H hydrate from the original ther- hydrocarbons,as a consequence of active gas hydrate crystallizationduring migration in the sub-seafloor. If such hydrate crys- hydro- hydrocarbons would bevious field observations, however, little carbon isotopicdrate, including C hydrocarbons (Brooks ., 1988). It is difficult to explain the ob-hydrate formation, and thus we should assume alterna-bial activity. James and Burns (1984) found that C and alkanes were significantly and C alkanes. In Fig. 5, of the reciprocal of each carbon number, together withvalues for the Sirus-1 Exmouth Plateau, offshore Aus-suggested (James and Burns, 1984). The alteration wouldprobably be due to anaerobic microorganisms (Perry,1980; Stephens and Dalton, 1986; Ashraf ., 2003). The two sets of results coincide strik-levels of biodegradation in lower-temperature (reservoirs (Wenger sumption. The kinetic isotope fractionation during aero- hydrocarbons has., 2008). Thus, (moder- (partly) in thermogenic hydrocarbons couldTo verify this hypothesis quantitatively, we estimated hydrocar- hydrocarbons, so that the observed concentra- 3 vol% and (0.4 ppmv and , respectively, showing a reverse correlation 1)) in carbon iso- during the microbial alteration was 3 1,which corresponds to the KIE during the atmospheric (Tsunogai and C during the microbial altera- 0.7 and 1.5 0.5, respectively, which cor-tion effect of KIEs by unreacted carbon in a molecule(Tsunogai ., 1999). The assumed KIE for C corre-Under these assumptions, we estimated the initial C) from the concentrations and hydrocarbons in the bubbles, by correcting the micro-),(7) and C were the initial and final concentrations,respectively, and and were the initial and fi-C values, respectively. The estimated initial con- hydrocarbons corresponds to the612 (maximum). All the initial C (signifi-hydrocarbons can explain the observed molecular and iso- (significantly), CHe ratio of coexisting helium. Thein converting organic matter to hydrocarbonsHe ratios of helium in Deep-sea seeping methane bubbles in the Ryukyu forearc471 denotes the (Lupton, 1983; Mamyrin and Tolstikhin, 1984) and583F, and 584; Sano and Wakita, 1987) (open triangles).To exclude samples contaminated by air to a significantNe ratios exceeding 0.5. In addition, boldSimilarly, the mixing with seawater is presented for bothNe ratios, we can obtain theHe ratios for the bubbles and sediments and 0.15R, respectively, through removingNe in the samples was derived from the seawater.is lower than the ratios for the mantle or seawater, it isdifficult to explain the composition of Kuroshima bub- contribution of radiogenic helium in the crust/sediments. Although theNe ratio of the bubbles through the mixing of seawaterThus, there must be a contribution of mantle helium tobubbles at the Taketomi spring. The Taketomi submarine30 m depth) is 20 km northwest of the knolland on the same forearc seafloor of the southern Ryukyu (7080%) are discharged together with hot water having a ratios of 10, theparable to values for Kuroshima Knoll. The Late MioceneYaeyama Group, which includes coal and volcanic prod-ucts, is assumed to be the source rock of the hydrocar-bons in the Taketomi spring (Kaneshima He-enriched helium have been reported forsubaerial natural gas seepages in western Taiwan (Sano., 1988; Yang 1986; Yang ., 2004; You originated from the Miocene Mushan and WuchihshanFormations (Lu and Lin, 1986). The contribution of man-thermocatalytic decomposition of organic matter is ubiq-., 1998; Takeuchi ., 2007). The sedi-mentary rocks of Miocene Yaeyama Group, however, usu-ally underlies the Shimajiri Group in the southern Ryukyuislands (Tsuburaya and Sato, 1985). Although we couldnot find the Miocene Yaeyama Group on the surface, wecan reasonably assume the Miocene Yaeyama Group ex-bles in Kuroshima Knoll. Because the southern Ryukyu1968; Tiba and Saito, 1974), both the initial generation hydrocarbons, as presented in the previousSub-seafloor anaerobic oxidation of methane in the bub- 0.5 enrich- enrichment at maximum (Ta- in the bottom fluid samples corre- from ambient water as for the causein the bottom fluid (Table 2) were 10 times those inambient bottom water (Fig. 6). We conclude that kinetic must be responsible for the in the bottom fluid samples is thethe concentration. However, we could not find a clear 472U. Tsunogai in the bottom fluid samples (Table 2). Further- times that of ambienthad progressed in the fluid before dilution with the sur-rounding ambient water. We conclude that the majorityof the oxidation progressed under the seafloor. This is in up to 20 withinpore water of the seeping site. The CH in the liquid sam- oxidized anaerobicallyduring diffusive migration in the sedimentary layer., 1986; Tsunogai with CH, it might be limited to a thin surface layer,knoll as observed at another cold seepage site (Kulm and in the bubbles suggest that theyhad not undergone anaerobic oxidation, probably owingsedimentary layer. The trace concentration of CO in thebubbles also supports that there was minimum oxidationAs already discussed, the contribution of gases de-rived from the dissociation of sub-seafloor methane hy-He ratios and bubbles highly deviate from the ratios for seawater, it isthe bubbles in the knoll, the helium in the bubbles musthave been trapped in the methane hydrate as well, together in the bubbles. However, it is difficult to ex-plain the helium enrichment in the bubbles derived fromKennedy, 2000; Winckler Cascadia Margin, for instance, the ratios werefrom 0.0003 to 0.009 ppmv (Winckler ppmv. The dissociation of methane hydrate cannot be theonly source of the bubbles in the knoll. Thus, if we per-sist with the contribution of methane hydrate, we must ratios in the Kuroshima bubbles can be clas- ratios of ther-mogenic natural gases in and around Japan islands (Wakitacharges of CH bubbles at many sites of the world ocean,study. Both the present progression of global climate seeping sitesseem to support the assumption. Most of the submarinemassive hydrates, however, seems to be stable at present seeping activityon the seafloor. Regarding the origin of the seafloor seep-ing bubbles, we also need to take into account other pos- and the reciprocal concentration in the effluent plume (column watersamples taken from 500 to 700 m depths). The solid line is theleast-squares fitting of the data. Deep-sea seeping methane bubbles in the Ryukyu forearc473seafloor gas reservoirs, as presented in this study.We observed a clear enrichment of CH in the watercolumn at depths between 536 and 694 m (Fig. 6). On theother hand, we did not find any CH enrichment shal-lower than 500 m at the stations. All the rising bubbles inseafloor. This is consistent with observations of otherbubbles seeping from seafloor (Suess ., 1999; Valen-than the maximum mixed layer depth (. 100 m) in theTo clarify whether the CH decrease in the plume canseawater, the measured concentra-water samples taken from 500 to 700 m depths) (Fig. 7).shown in the figure. The linear correlation suggests that and CH in back- in the plume, and the is uniform during the mixing(Tsunogai ., 1998, 2000, 2005). That is, aerobic mi- in the plume, and thus CH is diluted rapidly byeddy diffusion rather than oxidation in the water column.have dissolved within 140 m of the seafloor. After disso-lution, the plume spreads horizontally along with the sur-tion is reduced through dilution by eddy diffusion, ratherQuantification of sub-seafloor anaerobic oxidation seeping from the knoll (Tsunogai ., 1998), which can be considered the weighted aver-ter column. Assuming the prior to at Kuroshima Knoll.Assuming the kinetic isotope effect due to the anaero- oxidation (Tsunogai canbe calculated from the equation (7). Using this equation, we estimated the aver- to be 0.82. Thus, about 20% of CH has beenbubbles were supplied directly from deep gas reservoirsproduced by thermogenic processes in the Miocene. Themicrobial destruction during the storage in the gas reser-voir. In addition, the anaerobic oxidation of CH within20% of CH on average, until seepage into ocean watercolumn. After the seepage, all rising bubbles dissolvedinto the ocean water column within 140 m of the seafloor.After the dissolution, the plume spreads horizontally alongeddy diffusion, rather than by oxidation.We are grateful to anonymous reviewerssightful suggestions. We are also grateful to Daniele L. Pintifor his help in improving the English of this paper. We wouldlike to thank the officers, crews, and scientists on the cruises ofR/V (JAMSTEC) NT02-08 and R/V Tansei-maru(JAMSTEC) KT05-26. This work was supported by severalTechnology (MEXT) Grant-in-Aid for Scientific Research inPriority Areas under Grant No. 18067001 (W-PASS), the MEXT Project. Finally, U.T. wishes to acknowledge a great debtAka, F. T., Kusakabe, M., Nagao, K. and Tanyileke, G. (2001)Noble gas isotopic compositions and water chemistry ofsoda springs from the islands of Bioko, Sao Tome andAnnobon, along with Cameroon Volcanic Line, West Af-Ashraf, W., Mihdhir, A. and Murrell, J. C. (1994) BacterialFEMS Microbiol. LettBains, S., Corfield, R. M. and Norris, R. D. (1999) MechanismsBernard, B., Brooks, J. M. and Sackett, W. M. (1977) AOffshore Technology Confer- 474U. Tsunogai derstanding source distribution and preservation of Austral-APPEA JBouchard, D., Hunkeler, D. and Hohener, P. (2008) Carbon iso-Org.Brooks, J. M., Cox, H. B., Bryant, W. R., Kennicutt, M. C., II,Mann, R. G. and McDonald, T. J. (1986) Association of gasOrg.Carpenter, J. H. (1965) The Chesapeake Bay Institute techniquefor the Winkler dissolved oxygen method. Chung, H. M., Gormly, J. R. and Squires, R. M. (1988) Originof gaseous hydrocarbons in subsurface environments: Theo-retical considerations of carbon isotope distribution. Coleman, M. L., Shepard, T. J., Durham, J. J., Rouse, J. E. andDe Angelis, M. A., Lilley, M. D., Olson, E. J. and Baross, J. A., 11691186.Dickens, G. R. and Kennedy, B. M. (2000) Noble gases in meth-Proc. ODP, Sci. ResultsDickens, G. R., Castillo, M. M. and Walker, J. C. G. (1997) Aeffects of massive dissociation of oceanic methane hydrates.Du, J., Jin, Z., Xie, H., Bai, L. and Liu, W. (2003) Stable car-Org. GeochemEtiope, G. and Milkov, A. V. (2004) A new estimate of globalEnviron. GeolEtiope, G., Feyzullayev, A. and Baciu, C. L. (2009) Terrestrialmethane seeps and mud volcanoes: A global perspective ofMar. Pet. GeolGamo, T., Sakai, H., Ishibashi, J., Oomori, T., Chiba, H.,Shitashima, K., Nakashima, K., Tanaka, Y. and Masuda, H.the CLAM Site, Mid Okinawa Trough, inferred from theirHead, I. M., Jones, D. M. and Larter, S. R. (2003) BiologicalNatureHeeschen, K. U., Trhu, A. M., Collier, R. W., Suess, E. andRehder, G. (2003) Distribution and height of methane bub-ble plumes on the Cascadia Margin characterized by acous-Hirayama, H., Sunamura, M., Takai, K., Nunoura, T., Noguchi,T., Oida, H., Furushima, Y., Yamamoto, H., Oomori, T. anda coral reef off Taketomi Island, Japan. Appl. Environ.MicrobiolIjiri, A., Tsunogai, U. and Gamo, T. (2003) Simple method for reagent. Rapid Commun. Mass SpectromInagaki, F., Tsunogai, U., Suzuki, M., Kosaka, A., Machiyama,H., Takai, K., Nunoura, T., Nealson, K. H. and Horikoshi,Knoll, the Southern Ryukyu Arc, by analyzing and 16S rRNA Genes. Appl. Environ. Microbiolsis. Contribution of Working Group I to the Fourth Assess-ment Report of the Intergovernmental Panel on ClimateJames, A. T. and Burns, B. J. (1984) Microbial alteration ofJudd, A. G. (2004) Natural seabed gas seeps as sources of at-Environ. GeolKaneshima, K., Taira, H., Tokuyama, A., Ossaka, J. and Kimura,out of sea bottom at coastal area of Taketomi-jima. Fac. Sci. Univ. RyukyusKawano, Y. and Ueda, Y. (1966) K-A dating on the igneousJ. Japan. Assoc. Min. Pet. Econ. Geol211 (in Japa-Mar. ChemKinnaman, F. S., Valentine, D. L. and Tyler, S. C. (2007) Car-Geochim. Cosmochim. ActaKomatsu, D. D., Tsunogai, U., Yamaguchi, J. and Nakagawa,F. (2005) Stable carbon isotopic analysis of atmosphericSpectromKulm, L. D. and Suess, E. (1990) Relationship between car-Kulm, L. D., Suess, E., Moore, J. C., Carson, B., Lewis, B. T.,Ritger, S. D., Kadko, D. C., Thornburg, T. M., Embley, R.W., Rugh, W. D., Massoth, G. J., Langseth, M. G., Cochrane,Kvenvolden, K. A. (1988) Methane hydrate and global climate. Deep-sea seeping methane bubbles in the Ryukyu forearc475Kvenvolden, K. A. (1995) A review of the geochemistry ofOrg. GeochemLu, D. L. and Lin, J. T. (1986) Carbon and hydrogen isotopesof natural gas in western Taiwan. Petrol. Geol. TaiwanLupton, J. E. (1983) Terrestrial inert gases: Isotope tracer stud-Rev. Earth Planet. SciMacDonald, G. J. F. (1990) Role of methane clathrates in pastMachiyama, H., Matsumoto, T., Matsumoto, R., Hattori, M.,Okano, M., Iwase, R. and Tomaru, H. (2001) Outline ofShinkai 2000 dive surveys on the Kuroshima Knoll, offMamyrin, B. A. and Tolstikhin, I. N. (1984) Nature. Elsevier Science, The Netherlands, 273 pp.Mango, F. D. (2001) Methane concentrations in natural gas:Org. GeochemMango, F. D. and Elrod, L. W. (1999) The carbon isotoipic com-position of catalytic gas: A comparative analysis with natu-Geochim. Cosmochim. Acta1106.Matsuda, J., Matsumoto, T., Sumino, H., Nagao, K., Yamamoto,J., Miura, Y., Kaneoka, I., Takahata, N. and Sano, Y. (2002)Matsumoto, T., Kimura, M., Nishida, S. and Ono, T. (1998)ered on the Kuroshima Knoll south of Yaeyama IslandsMau, S., Valentine, D. L., Clark, J. F., Reed, J., Camilli, R. andWashburn, L. (2007) Dissolved methane distributions andair-sea flux in the plume of a massive seep field, Coal OilMerewether, R., Olsson, M. S. and Lonsdale, P. (1985) Acous-Milkov, A. V., Claypool, G. E., Lee, Y.-J., Torres, M. E.,Borowski, W. S., Tomaru, H., Sassen, R. and Long, P. E.surface gases indicate gas hydrate occurrence in marinesediments at southern Hydrate Ridge offshore Oregon. Org.Bull. Fac. Edu., Kagoshima Univ. Nat. SciNagy, B. (1960) Review of the chromatographic theoryGeochim. Cosmochim. ActaNakagawa, F., Tsunogai, U., Yoshida, N. and Adams, D. D.drocarbons in marginal marine sediments. rine Hydrogeology (Taniguchi, M., Wang, K. and Gamo,T., eds.), 141150, Elsevier Science, The Netherlands.Nakagawa, H. (1983) Outline of Cenozoic history of the RyukyuNishimura, R., Tsunogai, U., Ishibashi, J., Wakita, H. and Nojiri,Y. (1999) Origin of Towada, Japan. Okutani, T., Fujikura, K. and Sasaki, T. (2004) Two new spe- (Bivalvia: Mytilidae) from methaneseeps on the Kuroshima knoll off the Yaeyama Islands,Venus110.Oomori, T. (1987) Chemical compositions of submarine hotTaketomi-jima at southern part of the Ryukyu Island Arc,Oomori, T., Tanahara, A., Taira, H., Kimura, M., Kato, Y. andKiyosu, Y. (1993) Geochemical study on Taketomi subma-Oremland, R. S., Whiticar, M. J., Strohmaier, F. E. and Kiene,R. P. (1988) Bacterial ethane formation from reduced,Paull, C. K., Ussler, W., III, Borowski, W. S. and Spiess, F. N.rise: Associations with gas hydrates. Perry, J. (1980) Propane utilization by microorganisms. Adv.Appl. Microbiol115.Poreda, R. J., Jeffrey, A. W. A., Kaplan, I. R. and Craig, H.NatureSaegusa, S., Tsunogai, U., Nakagawa, F. and Kaneko, S. (2006)WHATS II for Japanese submersibles/ROVs. Sakai, H., Smith, J. W., Kaplan, I. R. and Petrowski, C. (1976)Sakai, H., Gamo, T., Kim, E.-S., Tsutsumi, M., Tanaka, T.,Ishibashi, J., Wakita, H., Yamano, M. and Oomori, T. (1990)Venting of carbon dioxide-rich fluid and hydrate formationin mid-Okinawa Trough backarc basin. Sano, Y. and Wakita, H. (1987) Helium isotopes and heat flowon the ocean floor. Sano, Y., Wakita, H. and Huang, C.-W. (1986) Helium flux in aern Taiwan. NatureSassen, R., Sweet, S. T., Milkov, A. V., DeFreitas, D. A. andKennicutt, M. C., II (2001) Thermogenic vent gas and gas110.Sauter, E. J., Muyakshin, S. I., Charlou, J. L., Schlter, M.,Boetius, A., Jerosch, K., Damm, E., Foucher, J.-P. and 476U. Tsunogai Sills, G. C. and Wheeler, S. J. (1992) The significance of gasfor offshore operations. Silverman, M. P. and Oyama, V. I. (1968) Automatic apparatusSloan, E. D., Jr. (1998) Org. GeochemStephens, G. M. and Dalton, H. (1986) The role of the terminalJ. Gen. MicrobiolStrapoc, D., Mastalerz, M., Eble, C. and Schimmelmann, A.Org. GeochemSuess, E., Torres, M. E., Bohrmann, G., Collier, R. W., Greinert,J., Linke, P., Rehder, G., Trehu, A., Wallmann, K., Winckler,G. and Zuleger, E. (1999) Gas hydrate destabilization: en-hanced dewatering, benthic material turnover and largemethane plumes at the Cascadia convergent margin. renssen, A., Jamtveit, B.,Myklebust, R., Eidem, T. R. and Rey, S. S. (2004) ReleaseNatureTakeuchi, R., Matsumoto, R., Ogihara, S. and Machiyama, H. on theKuroshima Knoll, Ryukyu islands, Japan. Taylor, S. W., Sherwood, B. and Wassenaar, L. I. (2000) Bacte-riogenic ethane in near-surface aquifers: Implications forEnviron. Sci. TechnolTiba, T. and Saito, Y. (1974) A note on the volcanic rocks ofIriomote-jima, Ryukyu Islands. Mem. Nat. Sci. Mus., TokyoTsuburaya, H. and Sato, T. (1985) MITI Miyakojima oki well.J. Japan. Assoc. Petrol. TechTsunogai, U., Ishibashi, J., Wakita, H. and Gamo, T. (1998)Methane-rich plumes in Suruga Trough (Japan) and theirEarth Planet. Sci. LettTsunogai, U., Yoshida, N. and Gamo, T. (1999) Carbon iso- hydrocarbons and methyl chlo-Tsunogai, U., Yoshida, N., Ishibashi, J. and Gamo, T. (2000)Tsunogai, U., Yoshida, N. and Gamo, T. (2002a) Carbon iso-seafloor at Nankai Trough. Mar. GeolTsunogai, U., Nakagawa, F., Komatsu, D. D. and Gamo, T.Tsunogai, U., Toki, T., Nakayama, N., Gamo, T., Kato, H. andKaneko, S. (2003) WHATS: a new multi-bottle gas-tightTsunogai, U., Nakagawa, F., Gamo, T. and Ishibashi, J. (2005)Urabe, A., Tominaga, T., Nakamura, Y. and Wakita, H. (1985), 11Valentine, D. L., Blanton, D. C., Reeburgh, W. S. and Kastner,M. (2001) Water column methane oxidation adjacent to anGeochim. Cosmochim. ActaWakita, H. and Sano, Y. (1983) NatureWenger, L. M., Davis, C. L. and Isaksen, G. H. (2002) Multiplecontrols on petroleum biodegradation and impact on oilquality. SPE Reservoir Evaluation & EngineeringWhiticar, M. J., Faber, E. and Schoell, M. (1986) Biogenicmethane formation in marine and freshwater environments: reduction vs. acetate fermentation-isotope evidence.Geochim. Cosmochim. ActaWinckler, G., Aeschbach-Hertig, W., Holocher, J., Kipfer, R.,Levin, I., Poss, C., Rehder, G., Suess, E. and Schlosser, P.Xu, S., Nakai, S., Wakita, H. and Wang, X. (1995) Mantle-Geochim. Cosmochim. ActaYang, T. F., Chen, C.-H., Tien, R. L., Song, S. R. and Liu, T. K.East Taiwan after arc-continent collision: fission-track dataRadiation MeasurementsYang, T. F., Yeh, G.-H., Fu, C.-C., Wang, C.-C., Lan, T.-F., Lee,H.-F., Chen, C.-H., Walia, V. and Sung, Q.-C. (2004) Com-in Taiwan. Environ. Geol1011.You, C.-F., Gieskes, J. M., Lee, T., Yui, T.-F. and Chen, H.-W.(2004) Geochemistry of mud volcano fluids in the Taiwan